The present invention relates generally to reception and processing of spread spectrum signals. More particularly the invention relates to a spread spectrum receiver according to the preamble of claim 1 and a method according to the preamble of claim 9. The invention also relates to a computer program product according to claim 17 and a computer readable medium according to claim 18.
Spread spectrum transmission solutions are becoming increasingly important, for instance in global navigation satellite systems (GNSS). Presently, the Global Positioning System (GPS; U.S. Government) is the dominant system, however alternative systems are expected to gain increased importance in the future. So far, the Global Orbiting Navigation Satellite System (GLONASS; Russian Federation Ministry of Defense) and the Galileo system (the European programme for global navigation services) constitute the major alternative GNSSs. Various systems also exist for enhancing the coverage, the availability and/or the quality of at least one GNSS in a specific region. The Quasi-Zenith Satellite System (QZSS; Advanced Space Business Corporation in Japan), the Wide Area Augmentation System (WAAS; The U.S. Federal Aviation Administration and the Department of Transportation) and European Geostationary Navigation Overlay Service (EGNOS; a joint project of the European Space Agency, the European Commission and Eurocontrol—the European Organisation for the Safety of Air Navigation) represent examples of such augmentation systems for GPS, and in the latter case GPS and GLONASS.
Unfortunately, the dissimilarities in the frequency bands, and especially the signal formats used in the different systems, result in the situation that a signal receiver adapted for one system is generally not able to receive and process signals from sources belonging to a different system. Thus, multiple receiver chains, or one receiver chain with plural signal paths, are required to enable reception of signals from more than one type of system. Including more than one receiver chain in a single device renders the device expensive, bulky and/or heavy. Therefore, a programmable software receiver solution is desired, which enables processing of many signal formats in one processor, e.g. a CPU (central processing unit) or a DSP (digital signal processor). Namely, in such a design, it is possible to adapt the signal processing principles to a plurality of signal formats. A software-based GNSS receiver is also advantageous in that this kind of receiver may co-exist efficiently with other types of signal receivers, signal processing devices and/or software applications, for example in a laptop computer, a mobile telephone or a PDA (Personal Digital Assistant).
However, software receiver implementations are associated with one important drawback in comparison with corresponding hardware designs. Namely, a software implementation running on a generic microprocessor is typically less energy efficient (in terms of energy or power per output data) than one running on a dedicated hardware implementation, e.g. represented by an ASIC (Application Specific Integrated Circuit).
For reasons mentioned above, it is advantageous to integrate a software-based receiver into a portable/handheld device, provided such integration addresses the additional challenge of these devices' typically limited battery capacity. Thus, it is important to optimize the use of the power resources as well. With these things in mind, we will now briefly discuss the prior art in this area.
U.S. Pat. No. 6,710,578 discloses a method for power resource management in a hardware-based portable communication device, such as a radiotelephone. The device may be operated in a plurality of operational modes, and before entry into a particular user-selected mode, an estimate of the available power is calculated. It is then predicted whether these resources are sufficient for the selected mode, and if it is estimated that the resources are insufficient, the operation of the device is restricted with respect to at least one operational mode. However, the selection of the signal sources being used is not influenced by these measurements. Furthermore, the solution is entirely focused on a hardware implementation.
U.S. Pat. No. 6,727,850 describes a method and an apparatus for selecting optimal satellites to locate an object. A satellite list including the coordinates of the visible satellites is generated. Then, the satellites having the highest redundancy are eliminated from the list, such that an intended number of satellites remain on the list. Thereby, a required computation volume to produce the list becomes relatively low. Here, the redundancy is defined by a degree of overlap that a satellite has with other satellites on the list. However, besides this redundancy measure, no quality-related parameter influences the choice of satellites used by the receiver. Furthermore, the design presumes a hardware implementation.
The published U.S. patent application 2005/0140545 reveals a GPS receiver having a software-implemented correlator, which is adapted to render possible seamless integration of multiple technologies without any compromise in performance levels and without the need for customized hardware. Allegedly, the design also reduces the power consumption as a result of fewer hardware components and the ability to change the sampling frequency. Nevertheless, optimization of power resources is not used in any way in the space vehicle (SV) selection algorithm.